1. Technical Field
The present invention relates generally to circuits for correcting the power factor of rectifier circuits, and in particular to those power factor correction circuits used in electronic ballasts.
2. Background Art
Typical prior art regulated power factor control schemes employ an integrated circuit, along with several external components, to control the duty cycle of a power switching transistor. The duty cycle of the power switch is used to control the input current waveform so that it approximates the shape of the input voltage waveform. The output of the power factor controller is stored in a bulk DC capacitor in order to provide a relatively constant DC power supply for the ballast. An example of this is taught in U.S. Pat. No. 5,177,408 to Marques. The integrated circuits used in prior-art power factor controllers generally include a pulse-width modulator (PWM) circuit, a high-gain error amplifier, and various other circuits. A prior art example of an integrated circuit PWM controller with a feedforward input is the Unitrode UC1841. In that circuit, feedforward is implemented by adjusting the amplitude of the internal ramp generator in response to the voltage at the Vin SENSE pin.